Currently, glass substrates are mainly used as a substrate in the field of electronic devices such as flat panel displays and electronic papers. However, glass substrates are disadvantageously heavy and brittle. Therefore, glass substrates are not always an ideal substrate. On this account, studies have been actively made so as to provide a flexible device in which a substrate made of glass is replaced by a substrate made of a polymer material. However, many of techniques for providing such a flexible device require new production techniques and apparatuses. Accordingly, a flexible device in which a polymer material is used has not yet been mass-produced.
Meanwhile, recently, there has been a proposal to produce, by use of an ordinary process for a case where glass substrates are used, a flexible device with use of a laminate in which a polyimide resin layer is formed on a glass substrate (Non-Patent Literature 1). This method is proposed as a shortcut for efficiently mass-producing flexible devices. In a process in which such a laminate is used, the flexible device is obtained by separating the polyimide resin layer from the glass substrate in a final step.
In the process, the laminate is required to have smoothness and low warpage for favorable handling. In other words, the polyimide resin layer of the laminate is required to have a linear expansion coefficient that is substantially the same level as that of glass. Note that soda-lime glass and alkali-free glass are generally used for glass substrates and soda-lime glass has a linear expansion coefficient of approximately 8 ppm/° C. to 9 ppm/° C. while alkali-free glass has a linear expansion coefficient of approximately 3 ppm/° C. to 5 ppm/° C. Further, a processing temperature in production of an amorphous silicon thin film transistor reaches at maximum 300° C. to 350° C. A material suitable for such a process is inevitably limited because a linear expansion coefficient of a general polyimide is greater than that of glass. For example, Patent Literature 1 discloses a method in which a laminate is obtained by (i) flow-casting, on an inorganic substrate, a solution of a polyimide precursor obtained from (a) 3,3′,4,4′-biphenyltetracarboxylic dianhydride and (b) paraphenylenediamine or 4,4″ diaminoparaterphenyl and (ii) subjecting the solution to thermal imidization. Meanwhile, in a case where a polyimide precursor having a specific structure is formed into a film on an inorganic substrate and then subjected to thermal imidization which is carried out by increasing a temperature at a certain rate or higher, a resultant polyimide film may peel off from the inorganic substrate. On this account, the inorganic substrate is surface-treated (Non-Patent Literature 2) or a silane coupling agent having an amino group and/or an acid anhydride group is added to a polyimide precursor solution (Patent Literatures 2 and 3), for the purpose of improving adhesion between polyimide and the inorganic substrate.